Thermoregulatory tactics and water balance of flying metander Centris caesalpiniae males

Many solitary bees form nesting aggregations where males compete for unmated, emerging females. For the dimorphic Centris caesalpiniae males, large morphs (metanders) dig for females at ground-level, flying occasionally, while the small morph hovers and flies. At a mate-aggregation site, air temperatures varied from 17 ºC to 37 ºC across and within days; operative temperatures of the metanders reached 50 ºC on the ground when mating activity ceased. Body segment temperatures increased with air temperature for bees both on the ground and in flight, indicating modest thermoregulation. Abdominal mass of metanders scaled strongly hypermetrically (slope of log/log plot = 1.6), requiring flight metabolic rates per gram of thorax to increase with mass. Water loss and the abdominal temperature excess ratio increased with air temperature during flight, suggesting that evaporative water loss and heat-dumping from the abdomen contribute to thermoregulation while flying. However, on the ground, head temperature excess ratios decreased, suggesting that evaporative water loss from the head is important. Metabolic water production is about half of water loss rates, and, when flying, metanders lose about 17% of their body water per hour. No one has observed metanders feeding or drinking, and several hours of flight would likely cause a lethal loss of body water. This suggests that water loss constrains the activity of metanders as the day warms to hazardous temperatures in the desert. Supported by 2019-2020 Fulbright Student Program and USDA 2017-68004-26322.